Restore Cirrus-CI boot smoke test

Restore Cirrus-CI boot smoke test

This reverts commit a7d593dd1da27833b5384349700bc3c7bcae6aad.
We now use compute_engine_instance which allows us to specify a custom
disk size.  Also go back to using the default qemu version (rather than
qemu42 or qemu-devel) as any issues were fixed some time ago.

Reviewed by:	lwhsu, markj
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30082

(cherry picked from commit cec2682cd2910080951782391d7d7cd30c95c4ea)